The time scale is set in mdhd, and later validated in the
enclosing trak atom once all of its children have been parsed.

A loose mdhd atom outside of a trak atom could update the time
scale of the last stream without any validation.

Reported-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Cc: libav-stable@libav.org
Signed-off-by: Martin Storsjö <martin@martin.st>